Position sensing apparatus with remote electronics for harsh environments

ABSTRACT

A sensing apparatus for use in harsh environments to measure a target characteristic. The apparatus has a sensing element formed as a section of a coupled slow-wave structure including at least two impedance conductors each curled into a helix with opposing directions of winding around a dielectric base to form a resonator. The sensing element provides as an output signal a digital frequency that depends on the value of the measured characteristic. A target tube moves over the sensing element, covering and uncovering portions of the sensing element. An electronics module receives the output signal and displays the measured characteristic. A separate coaxial cable is connected to each impedance conductor on one end and to the electronics module on the other end, with the length of the coaxial cables separating the electronics module from the sensing element by a distance sufficient to avoid exposing the electronics module to the harsh environments.

RELATED APPLICATION

This application claims the benefit of priority to U.S. ProvisionalPatent Application No. 62/856,886 filed on Jun. 4, 2019, the contents ofwhich are incorporated in this application by reference.

TECHNICAL FIELD

The present invention relates generally to sensors that use a sensingelement and electronic circuits in performing a sensing function tomeasure the value of a physical parameter, such as linear position,rotary position, gaps, or liquid levels, and that provide an electricaloutput indicative of the value of the measured parameter. Moreparticularly, the present invention relates to sensors that can operatein harsh environmental conditions.

BACKGROUND OF THE INVENTION

The usefulness of an RF (radio frequency) or microwave electromagneticfield for the purpose of linear position measurement is known. When anelectromagnetic field is excited near a movable object, the parametersof the electromagnetic field, such as resonant frequency, phase, oramplitude, vary with the change of position of the movable object. Theelectromagnetic field parameters may be converted into an electronicindication of position, displacement, velocity, or acceleration of themovable object. In particular, U.S. Pat. No. 6,819,208 discloses aferromagnetic actuator with a ferromagnetic circuit defining an axialtravel interval of a ferromagnetic armature for axially driving a rodbetween two extreme positions in which the armature bears against polesof the ferromagnetic circuit. A resilient return is provided to hold avalve at rest in a middle position between the extreme positions, and atleast one coil is carried by the circuit, enabling the armature to bebrought alternately into each of the two extreme positions. The rodcarries a radially magnetized bar of a length not less than the traveldistance of the armature, and the housing carries at least one magneticflux sensor placed in a zone having low exposure to the field created bythe coil or coils.

The application of slow-wave structures is also known for measuringliquid level and angular position. See U.S. Pat. Nos. 6,293,142 and6,393,912. These patents teach the significant decrease of physicaldimensions and resonant frequency of a sensing element. In thesepatents, a sensing element, fabricated as a section of a slow-wavestructure (SWS), is connected to a measuring circuit comprising an RFoscillator and a converter which converts the resonant frequency of thesensing element SWS into a level reading, in the first example, or to anangular position reading, in the second example.

The use of a SWS sensing element enables the control of electric andmagnetic field distribution in the transverse and in the longitudinaldirections. The use of coupled slow-wave structures makes it possible tosplit the electric and magnetic fields in the transverse direction.Splitting of the electric and magnetic fields can provide additionalslowing of the electromagnetic wave. Splitting them in the transversedirection can also enhance the dependence of the electromagnetic fieldparameters on the distance between the slow-wave structure and aconductive target.

Slow waves are electromagnetic waves propagating in one direction with aphase velocity V_(p) that is smaller than the velocity of light, c, in avacuum. The ratio c/V_(p) is called the deceleration factor or slowingfactor. It is designated as N. In most practical applications, slowedelectromagnetic waves are formed in slow-wave structures by coiling oneor two conductors, for example, into a helix or radial spiral, whichgeometrically increases the path length traveled by the wave. Such acurled conductor is called an “impedance conductor.” It is commonlypaired with another conductor that is not curled, called a “screenconductor.”

Additional deceleration, in addition to the geometric path length, canalso be obtained by positive electric and magnetic coupling in a coupledslow-wave structure. In this case, both conductors are coiled, and havethe configuration of mirror images flipped by 180 degrees relative to aplane of symmetry.

Slow-wave structure-based sensor elements are known. Slowing of anelectromagnetic wave leads to a reduction in the dimensions of a sensingelement for a given resonant frequency. Thus, by using the advantages ofelectrodynamic structures, a relatively small sensing element canoperate at relatively low frequencies. A lower operating frequency ismore convenient to generate, and more convenient for the conversioncircuit which produces a desired output signal. An operating frequencycan be chosen so that it is low enough to provide the above advantages,but still high enough to provide high accuracy and a high speed ofresponse.

The low electromagnetic losses at relatively low frequencies (a fewmegahertz (MHz) to tens of MHz) also helps to increase the accuracy andsensitivity of the measurement. In addition, slowing of theelectromagnetic wave leads to concentration of the energy in both thetransverse and longitudinal directions. This results in an increase insensitivity, proportional to the slowing factor.

Known devices can measure one or more parameters of an electromagneticfield. Some of the devices use one or two resonators, placed near amovable object for which the position is to be measured. Changes in theposition of the movable object result in changes of the electromagneticparameters of the resonator or resonators. The resonator or resonatorsis or are connected to a measuring circuit comprising an RF or microwavesignal generator, which is used to excite an electromagnetic field.

In practice, four different types of sensors are known for measuring,among other parameters, linear or rotary position, gaps, and liquidlevels. These sensors are linear variable differential transformer(LVDT) sensors, linear variable inductive transducer (LVIT) sensors,magnetostrictive sensors, and sensors based on distributed impedancesensor technology (DIST). Sensors can sometimes be physicallypartitioned into a sensing part and a signal conditioning part, so thatthe sensing part can be exposed to a higher or lower temperature (orsometimes, to nuclear radiation) and the signal conditioning part canremain at a benign temperature (or at a lower radiation level). Each ofthe four sensor types are summarized below.

An LVDT sensor is a non-contact inductive sensor that has been in usefor decades. (The first potential LVDT was designed and built by MichaelFaraday in 1831 as part of his work on the development of the electricmotor. It is still on display. Practical versions became common duringWorld War II.) It is a variable transformer with a movable magneticcore. The LVDT sensor has three wound coils: one primary and twosecondary coils. A carefully calibrated AC voltage is injected into theprimary coil and then transferred to the secondary coils via thecoupling of the magnetic core, which is usually a soft iron rod. As theposition of the core changes, the output voltages in the secondary coilschange thus producing a measure of the position of the core. A dedicatedelectronic module (a signal processor) provides the input signal,measures the output signal, and provides a linear output aftercorrecting for attenuations and phase changes in the wires. Thiselectronic module can be located remotely from the coil, allowing theLVDT coil to operate in a harsh environment while the signal conditionercan be placed in a more benign location.

An LVDT sensor requires three to six power and signal wires between thecoil and the signal analyzer, however, and the connecting wires carrysinusoidal and quadrature sinusoidal waveforms at kilohertz (kHz)frequencies, and sometimes at relatively low voltage levels, while theamplitude of the voltage, as well as the phase, represents the signal.Therefore, the connections must be made using a shielded cable, toprevent interference from external magnetic fields, and must be usedwith a signal conditioner that is designed for that type of LVDT sensorand that is calibrated for exactly that particular LVDT sensor. An LVDTsensor also requires a dedicated signal processor for calibration. Ifthe signal processor is replaced, the sensor must be recalibrated.Because the output signal from an LVDT sensor is analog, system noiseinevitably limits its ultimate resolution.

An LVIT sensor typically consists of a single coil of wire with amoveable permeable core. The inductance of such a device varies as thecore is moved in and out of the coil. If the coil is connected to anoscillator and driven below the resonant frequency of the circuit, theoutput voltage will vary with the movement of the core by using a simplecircuit. The resonance frequency is set by the capacitance andinductance of the circuit. A related configuration uses a second coilwith a permanent core and measures the ratio of the output of the twocoils which will also vary with moveable core position. Thisconfiguration is similar to an LVDT sensor, but currently requires theelectronics to be close to the coils. The main advantages of the LVITsensor are that its stroke-to-length is shorter than an LVDT sensor andthat it is simpler to manufacture.

Magnetostrictive sensors consist of a wire (the waveguide) that extendsover the length of the measurement range. A permanent ring magnet, theposition magnet, surrounds the wire and is attached to whatever ismoving. The wire is selected so that when an electrical interrogationpulse is applied to the wire, an ultrasonic strainwave is generated atthe location of the position magnet. The time elapsed between generationof the strainwave and its detection at one end of the waveguide isindicative of the measured position. This sensor can be very long(meters) but has a slow response time due to the large transit times ofacoustic pulses. The magnetostrictive sensor has one advantage overother sensors in that its resolution is 1 to 2 microns, independent ofthe length of the sensor. Such sensors do have environmental limitationsin regard to temperature, shock and vibration, however, and theirupper-temperature limit is only 100° C. This temperature limitation isdue to characteristics of the sensing element and not the electronics ofthe signal processor which is attached. The heart of themagnetostrictive sensor, the waveguide, is also susceptible to failurewhen used in high-shock and vibration applications. In some situationswhere space near the measurement point is limited, the signal processorcan be located remotely. Given the high power requirements of theinterrogation pulse and the weakness of the return signal, however, thedistance between the processor and sensing element is limited to a fewcentimeters and must be carefully shielded. As with the LVDT sensor, themagnetostrictive sensor and signal processor must be calibrated as aunit and replacement of the signal processor requires recalibration.

The sensing head of a magnetostrictive sensor can be separated from theconditioning electronics by a very short cable having wires for theinterrogation pulse and wires for the received signal pulse. But theinterrogation wires must carry voltage and current in the range of morethan ten volts and more than one ampere, while the signal wires carry asignal in the millivolt and micro amp range. This places limitations onthe length (a few centimeters) and type of cable (individually shieldedpairs) and requires a specialized electronics module that is designedfor the type of sensing element and that is calibrated for exactly thatparticular magnetostrictive sensor.

Like many conventional sensors, LVDT sensors, LVIT sensors, andmagnetostrictive sensors include magnetic materials, such as iron andnickel, and permanent magnets, which are sometimes not compatible withrequirements of a specific application. An LVDT sensor uses a core madefrom a ferromagnetic material (usually a nickel-iron alloy), andmagnetostrictive position sensors use a position magnet which is apermanent magnet, often a rare-earth magnet.

LVDT sensors, LVIT sensors, and magnetostrictive sensors typically use asensing element and an electronics module. The electronics module powersthe sensing element, conditions the signal provided from the sensingelement, and provides a desired output signal. The signal conditioningpart of the electronics module is designed specifically to provide thetype of power required, and to receive the type of signal delivered, bythe sensing element. The included analog and/or digital circuits forpowering and signal conditioning are generally somewhat complex, and therequired specialized components are difficult or impossible to find onthe market with maximum temperature capability of more than 125° C.

U.S. Pat. No. 7,216,054 describes sensors based on DIST. The DIST sensorconsists of a double coil wound on a round non-conductive rod (usuallyfiberglass). The wire is wound as a helix with a large pitch. Uponreaching the end of the shaft, the pitch is reversed and a returninghelix is laid over the first coil. In the electronics section, a simplecircuit consisting of a single transistor is connected to the ends ofthe coil, producing a resonant circuit that oscillates in the 2-4 MHzregion. The result is two coils in series with one having currentflowing clockwise and the other counterclockwise. The magnetic fields ofthese two coils are parallel to the sensor, point in oppositedirections, and cancel each other by the “right hand rule.” At the sametime the electric fields from these circulating currents areperpendicular to the rod and again by the “right hand rule” they areadditive. The resulting electromagnetic field outside the coil is thenmostly electric with a minimal magnetic component.

The frequency of this circuit is determined by the inductance (L) andcapacitance (C) of the coil. Operating a circuit at its resonancefrequency produces a very stable output (assuming that L and C remainconstant). The inductance of the DIST design is low and constant, due tothe few turns of the coil, but the ratio of capacitance to inductance ishigher than in an inductive sensor and is based on the interaction ofthe strong electric field of the sensor with any nearby conductivesurface. If a conductive surface begins to cover the coil, thecapacitance and resonant frequency of the sensing element changedramatically. Changes on the order of about one MHZ in resonantfrequency can be observed when the conductive surface moves the fulllength of the sensor. The change in frequency is linear with themovement of the conductive surface, can be transmitted great distanceswith no loss of information, and is easily converted to a digital signalfor further processing.

The output of the DIST sensor, as discussed above, is from a stableresonance circuit whose frequency is determined by the capacitancechanges of the moving conductive surface. A typical sensor will havealmost a one MHz change in frequency over the full range of motion.Because frequency can easily be measured to better than one part in onemillion, this translates to a reproducibility (and accuracy aftermapping) of better than one micron over sensor lengths up to one meter.The resolution of the sensor is sub-micron and is determined more by thequality of the frequency meter than the sensor itself.

In the DIST sensor there is no need to use extremely fine wire as withan LVDT sensor. Wire diameter is intentionally minimized in an LVDTsensor to produce a large inductance and a negligible capacitance, bothLVDT characteristics. In contrast, a DIST sensor can use a much heavier,flattened wire because the goal is to maximize capacitance. A morerobust wire becomes practical and is in fact advantageous. The heavierwire wound on a flexible rod can withstand extremely high levels ofshock and vibration and overcomes these susceptibilities of othersensors.

Having an output that is a digital frequency rather than a voltage hasmany advantages. The amplitude of a frequency signal in the DIST systemis not significant so long as it remains above the threshold ofdetection. The resistance of any metal wire changes with temperaturewhich translates to amplitude changes of any analog signal being sentthrough that wire. The frequency of the resonant circuit is notaffected, however, by the change in resistance of the wire. Typicalinductive sensors have analog outputs which make them susceptible to notonly temperature changes but noise, attenuations, and other distortions.They require that the system electronics be near to the sensor or haveextensive correction software to compensate for these errors. Because ofthese and other problems, it is difficult for an analog sensor to haveaccuracies better than one part in one thousand (0.1%) of full scale. Asdiscussed above, the DIST sensor accuracy is potentially one part in onemillion (0.0001%) of full scale. The frequency signal can be replicatedand sent over separate wires for redundancy or multiplexed giving itgreat flexibility in its mode of transmission. In addition, because thesignal analyzer is nothing more than a frequency counter, the DISTsensor does not lose its calibration if it becomes necessary to changethe electronics in the signal analysis portion.

The electronic section of the DIST sensor requires only a single activedevice (an inverter) which is available up to 150° C. (and for shorttime periods up to 225° C.) and high radiation versions. The outputfrequency can be piggybacked on the DC power so that only one wire isrequired and the receiver can be located remotely. All of this circuitry(along with an optional temperature sensor) easily fits on a very smallcircuit board. This configuration gives the DIST sensor the ability tooperate in harsh environments with only a single wire (plus a ground) totransmit information to a remote computer. In addition, the ability tohave a signal wire in excess of 10 meters in length, and longer withrepeaters, allows great flexibility in locating the signal analysiselectronics. In many applications, a frequency meter is already in placeand can make the measurement without additional complexity.

The single signal wire output from the one-wire DIST sensor is also verybeneficial when operating in a high-pressure environment. Conductortransitions from high to low pressure are expensive, vulnerable (usuallyglass seals), and could cause distortion of analog signals.

The DIST sensor uses a single dual-helix coil (not a ratio of coils asin the LVDT sensor) that is sensitive to temperature changes. This isbecause the temperature variation is due to electronic drift and theexpansion and contraction of the coil and the target tube. To compensatefor this change, the DIST sensor can have an integral temperature sensorto accommodate corrections. The temperature variations in the DISTsensor can also be minimized by using low coefficient of expansionmaterials such as ceramics and quartz for the coil and Al-Nicol metalfor the target tube.

The DIST sensor, having its signals multiplexed onto the single wireoutput, requires a simple passive filter network at the receiving end soas to direct the signals to their respective frequency meters. Inapplications that do not require operation in hazardous conditionsand/or have no space limitation, the conversion of the frequency tostandard outputs can be performed in an electronics module attached tothe sensor. This eliminates the need for external filter networks butlimits operation to temperatures below 125° C. and increases the size ofthe electronics package.

U.S. Pat. No. 8,692,541 teaches a sensing head having severalimprovements to the electromagnetic-type position sensor that has asensing element configured as a section of a coupled slow-wavestructure, used as a resonator, and coupled with an electricallyconductive movable target, such as that disclosed in the '054 patent.Accordingly, the term “sensing element” is descriptive of the purpose ofsuch a device, while the term “resonator” is descriptive of itselectrical function, and therefore these two terms are sometimes usedinterchangeably in this document. More specifically, the '541 patentteaches three improvements over the DIST sensor disclosed in the '054patent.

First, the sensing head is formed of a sensing element and a simplifiedelectronics module, the simplified electronics module including aresonant oscillator. A resonant oscillator is an oscillator having afrequency controllable by a resonator that is connected to it.Accordingly, the terms “oscillator” and “resonant oscillator” aresometimes used interchangeably in this document. The sensing element isthereby separated from the complex electronic circuitry that wouldotherwise be needed to provide an output that is a standard type for asensor. Electronic circuits, except for a very simple resonantoscillator circuit co-located with the sensing element, are removed, andthe sensing head has a variable frequency output signal instead. Thecombination of the sensing element and the resonant oscillator circuit,mounted into a much smaller housing, is called the sensing head. Thesimplicity of the resonant circuit allows high-temperature operation,because the simple components used can be found on the market withoperating temperatures of 225° C. or more. (Most electronic componentsoperate effectively up to 125° C. As the environmental temperatureincreases, however, the number and complexity of devices able to operatedrop rapidly while the cost and size increase dramatically. By 225° C.,only passive devices and very simple electronic components such asoperational amplifiers are available and they have limited lifetimesmeasured in hours. Similar but not as severe problems face operations atlower temperatures such as −60° C.) The amplitude of the variablefrequency output is not indicative of the signal, and so there is nodegradation of the accuracy of the signal, even with a separationdistance of more than 10 meters between the sensing head and a receivingdevice. Use of the simple resonant oscillator circuit in the sensorhead, rather than the use of normal signal conditioning electronics,allows the simplified electronics module to be very small. As a result,the usual diameter of about 4.8 centimeters for a conventional positionsensor having a single circuit board can be reduced to less than 2.0centimeters, still having a single circuit board. Use of the simplifiedelectronics module also enables the sourcing of components that canoperate in environments of higher levels of ionizing radiation.

The second improvement addresses high reliability applications. In suchapplications, multiple identical sensors have often been deployed tomake one measurement. With two sensors (a dual-redundant system), aslong as the two sensors agree, then the data are expected to beaccurate. If the two sensors report different readings, however, thenthe data from both sensors are suspect and should not be relied upon.With three sensors (a triple-redundant system), as long as at least twoof the sensors agree, then the system can continue to operate with thisvalue until a convenient time for service and replacement of the onesensor that disagrees. Dual and triple-redundant sensor systems havebeen deployed by installing two or three separate sensors, and thenmechanically coupling each of them to the same movable object, so thatthey each measure the position of the movable object. This mechanicalcoupling introduces errors due to differences in alignment, free-play,and other imperfect attributes of the mechanical couplings.

The sensing element of the sensing head taught by the '541 patentaddresses high reliability applications. The sensing element isconstructed so that up to three active elements (i.e., resonators) canbe located within the same physical space as a single sensor. For a dualsensing head, the two parts (resonators) of the sensing element can bepositioned coaxially, or linearly, with respect to one another. For atriple sensing head, two resonators of the sensing element arepositioned linearly, and the third resonator is mounted coaxially. Inboth cases, they all measure the position of the same movable object.

Finally, the sensing head taught by the '541 patent is connected withreceiving or conditioning equipment by only one wire, in addition to theusual chassis ground, common, or shield. The power and signal are bothcommunicated over the one wire. The signal is a variable frequency thatis impressed onto the one wire. The amplitude of the signal frequency isnot important, as long as the signal is detectable. The position or theposition and a temperature, or two positions, or two positions and atemperature, etc. can be impressed onto the one wire at the same time byfrequency division multiplexing (FDM). Each of the signals to beimpressed by FDM has its own individual frequency range of operation, sothe multiple FDM signals can be separated again as needed, by filteringover the respective frequency ranges. A demodulator circuit is alsotaught, as an example for separation of the FDM signals at the receivingend of a sensor system.

With only the sensing function contained within the sensing head, theexternal circuitry that is used for signal conditioning and/or analysiscan be made with a standardized calibration setting. Therefore, thesignal conditioning and/or analysis equipment can be changed if desired,without affecting the calibration of the sensor.

Known devices exhibit several problems. Some of the devices have lowaccuracy, sensitivity, and resolution at relatively low frequency,increasing only with a substantial increase in the operating frequency.An increase in frequency is accompanied, however, by an increase inelectromagnetic losses, such losses limiting the accuracy of themeasurement. It is also generally known that a higher operatingfrequency can increase the cost of the associated electronic circuitry.Some known devices therefore require complex and expensive equipment.Thus, there is a need in the art for an electromagnetic method andapparatus for monitoring position that has greater sensitivity,resolution, and lower cost.

To overcome the shortcomings of known devices, a new sensing apparatusis provided. An object of the present invention is to provide animproved apparatus that is relatively simple, economical, and compactand that can make accurate and reliable measurements of a target object.Such measurements can include, for example, linear distance, radial orrotary motion, proximity, and liquid levels. A related object is toavoid the need for compensation or complex circuitry to achieve accuratemeasurements. Another object is to provide an apparatus that can be maderedundant in a simple and reliable manner.

It is still another object of the present invention to provide animproved apparatus that can make accurate and reliable measurements inalmost any environment. Such environments can include, for example, highand low temperatures, high radiation levels, high pressure, vibration orshock, and caustic chemicals or steam. An additional object is toprovide an improved apparatus that is intrinsically safe.

BRIEF SUMMARY OF THE INVENTION

To achieve these and other objects, and in view of its purposes, thepresent invention provides an improved sensing apparatus. The sensingapparatus functions even in harsh environments to measure acharacteristic of an electrically conductive target. The apparatus has asensing element formed as a section of a coupled slow-wave structureincluding at least two impedance conductors each curled into a helix orspiral with opposing directions of winding around a dielectric base toform a resonator. The sensing element is connected by two coaxial cablesto a remote electronics module which includes electronic components tocreate a resonant circuit with the sensing element. A target tube isformed of an electrically conductive material and configured to moveover the sensing element, covering and uncovering portions of thesensing element. This will cause the frequency of the resonant circuitto change proportionally to the movement of the target tube. The lengthof the coaxial cables separates the electronics module from the sensingelement by a distance sufficient to avoid exposing the electronicsmodule to the harsh environments.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary, but are notrestrictive, of the invention.

BRIEF DESCRIPTION OF THE DRAWING

The invention is best understood from the following detailed descriptionwhen read in connection with the accompanying drawing. It is emphasizedthat, according to common practice, the various features of the drawingare not to scale. On the contrary, the dimensions of the variousfeatures are arbitrarily expanded or reduced for clarity. Included inthe drawing are the following figures:

FIG. 1 shows an electromagnetic linear position sensor in accordancewith U.S. Pat. No. 7,216,054;

FIG. 2 highlights the sensing element of the sensor shown in FIG. 1;

FIG. 3 illustrates the coaxial cable included as one component of theimproved sensor according to the present invention;

FIG. 4 is a representative block diagram illustrating a proximateelectronics module that is connected to the sensing element via shortwires in accordance with U.S. Pat. No. 8,692,541;

FIG. 5 is a representative block diagram illustrating a partiallyremoved electronics module that is connected directly to a coaxial cableand only indirectly to a resonant oscillator and then to the sensingelement;

FIG. 6 is a representative block diagram illustrating a remoteelectronics module that is connected to the sensing element via coaxialcables according to the present invention;

FIG. 7 shows an embodiment of the sensing element operating with avariable frequency according to the present invention;

FIG. 8 is a circuit diagram of another electronics module of the presentinvention;

FIG. 9 illustrates a dual-redundant sensor according to the presentinvention with the target tube removed to highlight the first and secondcoils;

FIG. 10 illustrates the dual-redundant sensor shown in FIG. 9 with thetarget tube in place;

FIG. 11 illustrates the dual-redundant sensor shown in FIGS. 9 and 10with the target tube at full extension;

FIG. 12 illustrates a triple-redundant sensor according to the presentinvention;

FIG. 13 graphically reflects the test results of a sensor constructed inaccordance with the present invention as a plot of output of the sensorin volts against the distance measured in inches;

FIG. 14 graphically reflects the non-linearity of the test results as aplot of full scale error percent against the distance measured ininches;

FIG. 15 is a graph depicting a series of three tests run from −70 to180° C. for a sensor constructed in accordance with the presentinvention as a plot of percent error of full scale against temperature;

FIG. 16 is a graph depicting high-temperature tests for a sensorconstructed in accordance with the present invention as a plot ofpercent error of full scale against temperature;

FIG. 17 is a graph that combines the data of the two graphs of FIGS. 15and 16;

FIG. 18A illustrates the helices of a modified sensor useful to measurethe gap of each turbine blade tip of a jet engine as it passes thesensor during operation of the jet engine;

FIG. 18B illustrates the helices shown in FIG. 18A as folded over eachother and imbedded into a ceramic body; and

FIG. 18C is a side view of the modified sensor as located in the cowlingwall with the turbine blade above the sensor and two coaxial cablesleading from the coils and extending out from the ceramic body.

DETAILED DESCRIPTION OF THE INVENTION

In a manner similar to inductive sensors, the sensor according to thisdisclosure follows the fundamental laws of physics concerning changingelectromagnetic fields. An electromagnetic wave will travel forever atthe speed of light in a straight line with no energy loss in a perfectvacuum. If the electromagnetic wave encounters any material, however,the energy and the direction of the electromagnetic wave will bechanged. The amount of the change can, in theory, be calculated by a setof well-known equations called Maxwell's equations. A position sensordetermines the change in the wave and from that can be used to calculatethe presence and position of an object in the path of the wave.Therefore, if an object moves into the path of the electromagnetic wave,the wave will be distorted in a predictable manner. If this distortionis measured, it is possible to determine the motion and position of theobject in the path of the wave, i.e., a position sensor.

Referring now to the drawing, in which like reference numbers refer tolike elements throughout the various figures that comprise the drawing,FIG. 1 shows an electromagnetic linear position sensor 100 in accordancewith U.S. Pat. No. 7,216,054. The sensor 100 has a housing 10 thatencloses an electronics module (not shown). The electronics module isusually manufactured as a printed circuit board having electroniccomponents mounted to its surfaces. A threaded area 20 providesstructure for mounting the position sensor 100 into a desired location.A sensing element 30 is constructed of an electrically insulated rod 32onto which are wound an inner helix 34 and an outer helix 36 ofelectrically conductive material. Although only the outer helix 36 canbe seen in FIG. 1, FIG. 2 illustrates the rod 32 and both helices 34, 36that form the sensing element 30. The inner helix 34 is wound onto theelectrically insulated rod 32 in the direction of arrow 34A; the outerhelix 36, in the direction of arrow 36B.

The two helices 34, 36 are arranged along the electrically insulated rod32 at a pitch, with a separating material to electrically insulate onehelix 34 from the other helix 36. The electrodynamic element thus formedis a part of a slow-wave structure. A cable 40 brings out electricalconnections from the electronics module that is within the housing 10,and contains, for example, four connection wires 50. In the example offour connection wires, they can be power, common, output voltage orcurrent, and calibration port. A target tube 60 is formed of anelectrically conductive material, such as an aluminum alloy, and ismoveable over the sensing element 30. As the target tube 60 moves towardthe threaded area 20, the target tube 60 covers up more and more of thelength of the sensing element 30. For example, the target tube 60 isshown in FIG. 1 at zero percent coverage of the sensing element 30, andwill cover the sensing element 30 by 100% when the target tube 60 movesto be adjacent to the threaded area 20. This movable range of 0 to 100%is the measuring range of the position sensor 100.

As summarized above, U.S. Pat. No. 8,692,541 offers several improvementsto the sensor 100 initially disclosed in U.S. Pat. No. 7,216,054. Oneimprovement is that the housing 10 is smaller and encloses a simplifiedelectronics module. Another improvement is that a pair of wires replacethe cable 40 and connection wires 50 for electrical connections thatwill provide power and support signal transmission. One wire of the pairis the power wire; the other conductor is a power return conductivepath, such as circuit common or case connection. The power return orcase connection is not required to be a separate wire, but may beconnected directly through contact with the housing 10.

Regardless of which electronics module and electrical connections areused, the helices 34, 36 of the sensing element 30 must be coupled withthe electronics module in order for the sensor 100 to operate as aposition sensor. The known sensor 100 places the electronics module inthe housing 10 which is located proximate the sensing element 30—asshown in FIG. 1—and thereby allows direct connection of the helices 34,36 to the electronics module and avoids the need for further wires tocouple the helices 34, 36 to the electronics module. If ordinary wiresof any significant length are used for this coupling purpose, they wouldbecome part of the coil and would distort the signal.

The problem with locating the housing 10 (and the enclosed electronicsmodule) proximate the sensing element 30 is that the electronics modulemust withstand the same environment as the sensing element 30. It wouldbe desirable to use the sensing element 30 in harsh environments,whether those harsh environments be high temperature (e.g., in excess of125° C.), low temperature (e.g., below −60° C.), high radiation, highpressure, caustic chemicals, vibration, shock, steam, or another adversecondition. It might be possible to include specialized electroniccomponents on the electronics module able to perform under some adverseenvironmental conditions, but such specialized electronic componentswould increase the size, cost, and complexity of the sensor 100, andwould have limited performance and lifetimes in the harshestenvironmental conditions. In addition, temperature and other sensorswould be needed to correct for the changes in signal due to these harshconditions.

A solution to that problem is provided by the inclusion of a readilyavailable, low-capacitance coaxial cable 200 in the sensor 100. As shownin FIG. 3, the coaxial cable 200 has an outer plastic sheath 210, awoven copper shield 220, an inner dielectric insulator 230, and a coppercore 240. The coaxial cable 200 is a type of electrical cable that hasan inner conductor (e.g., the copper core 240) surrounded by a tubularinsulating layer (e.g., the inner dielectric insulator 230), surroundedby a tubular conducting shield (e.g., the woven copper shield 220). Manycoaxial cables, like the example shown, also have an insulating outersheath or jacket such as the outer plastic sheath 210. The term“coaxial” comes from the inner conductor and the outer shield sharing ageometric axis. Coaxial cable was invented by English physicist,engineer, and mathematician Oliver Heaviside, who patented the design in1880. The coaxial cable 200 is a type of transmission line, used tocarry high frequency electrical signals with low losses. The coaxialcable 200 differs from other shielded cables because the dimensions ofthe cable and connectors are controlled to give a precise, constantconductor spacing, which is needed for it to function efficiently as atransmission line.

One coaxial cable 200 is connected to each one of the helices 34, 36;therefore, two coaxial cables 200 are included with the sensor 100. Theends of the coaxial cables 200 opposite the ends that are connected tothe helices 34, 36 are connected to the electronic components on theelectronics module. The electronics module is thus separated from thesensing element 30 by the length of the two coaxial cables 200 and neednot be enclosed in the housing 10. The sensing coil is essentially andeffectively extended using the coaxial cables 200. The length of thecoaxial cables 200 is typically the same for each coaxial cable 200.Typical lengths are about 10 feet (3 meters), 16 feet (5 meters), 20feet (6 meters), 50 feet (15 meters), or more. Thus, the improved sensor100 with the coaxial cables 200 has no electronic components (active orpassive) that need to be placed close to the sensing element 30.

The addition of the coaxial cables 200 to the sensor 100 allows forplacement of the sensing element 30 in a harsh environment or in alocation that is difficult to reach. Such placement is possible becauseall of the electronic components can be located at a remote site. Theremote site can be one which is not exposed to harsh conditions, thusavoiding any compromise in performance and allowing for more flexibilityin installing the sensor 100.

Because the helices 34, 36 of the senor coil are constructed ofhigh-temperature magnet wire, there is almost no problem in using thesensing element 30 in harsh environments between −70 and 200° C. Magnetwire is copper that is coated with an insulator to prevent shorting whenthe wires touch. It is the insulation that fails around 200° C., whichis why the sensor 100 uses ceramic strips on bare copper. The helices34, 36 are preferably made of relatively large, flat wires, as opposedto conventional small round wires, to obtain high capacitance. Coaxialcables 200 suitable for harsh environments are readily available in themarket.

The coaxial cables 200 do not affect the measurements obtained from thesensor 100. Although the coaxial cables 200 add capacitance to theresonant circuit, the added capacitance is a fixed amount that does notvary even if the coaxial cables 200 move around. The capacitance addedby the coaxial cables 200 does set a limit on the maximum length of thecoaxial cables 200 that can be used; there is a reduction in thefrequency change due to the added capacitance which will be the limitingfactor. The maximum length has been tested to about 20 feet (6 meters)but the limit is estimated to be in the hundreds of feet. Almost everyknown application would be covered by this amount of separation betweenthe sensing element 30 and the electronics.

It might be possible to place the coaxial cables 200 between the coiland the electronics of an LVIT sensor (as opposed to the sensor 100).Such placement would theoretically allow the use of remote electronicsin connection with the LVIT sensor. Because the coaxial cables 200cannot shield magnetic fields as well as they do electric fields,however, they are not likely to work as well with inductive sensors suchas the LVIT sensor. In addition, these sensors have analog signals sothere still would be attenuation and distortion over long cables due totemperature-induced resistance changes in the conductor wires. Thesereasons might explain why there is no known use of coaxial cables withLVIT sensors.

The various functions that are included within a typical electronicsmodule in order to operate the sensor 100 as a position sensor are shownin FIG. 4. FIG. 4 is a representative block diagram of a conventionalelectronics module 41 that is connected to the sensing element 30 via apair of short wires 47. Therefore, the conventional electronics module41 can be called a “proximate” electronics module 41 because it islocated proximate (i.e., within a few centimeters of and insubstantially the same environment as) the sensing element 30. Theelectronics module 41 is typically manufactured as a single printedcircuit board with electronic components mounted onto its surfaces. Theelectronics module 41 (including all of its components) is attacheddirectly to the sensing element 30.

As shown in FIG. 4, the electronics module 41 includes a resonantoscillator 42 that is connected directly, via the short wires 47, to thesensing element 30. (Components are connected or joined directly whenthere are no intervening elements between the directly joined orconnected components.) The resonant oscillator 42 oscillates, along withthe sensing element 30, at the resonant frequency of the sensing element30. The frequency depends on the value of a measured characteristic suchas position. This position frequency is sent as a first signal 43 to afrequency meter 44. The frequency meter 44 converts the first signal 43into a second signal 45 and sends the second signal 45 to amicroprocessor 46. The microprocessor 46 converts the second signal 45into either an output voltage (V) and current (I) or a digital number.The output signal indicates the position output 48 for the sensor 100.

The microprocessor 46 is a computer processor that incorporates thefunctions of a central processing unit (CPU) on a single integratedcircuit (IC), or sometimes up to eight integrated circuits. Themicroprocessor 46 is a multipurpose, clock-driven, register-based,digital IC that accepts binary data as input, processes it according toinstructions stored in its memory, and provides results (also in binaryform) as output. Microprocessors contain both combinational logic andsequential digital logic. Microprocessors operate on numbers and symbolsrepresented in the binary number system.

In the conventional electronics module 41, as described in the '541patent, power conditioning circuits ensure that transient voltages (suchas electrostatic discharge) or reversed connections will not damage thecircuitry. The power conditioning circuits also include a voltageregulator and a voltage inverter to provide proper voltages to variousparts of the electronics module 41. A typical regulated voltage is 3.3VDC (volts, direct current). The inverter supplies a slightly negativevoltage (e.g., about −1.0 VDC) so that the output amplifier of an outputsection can fully go down to 0.00 VDC when desired, with a positionsensor having a voltage output. Optionally, the output can be digitalformat, with a sufficient number of connection wires to support thechosen format.

A sensing section includes a connection for the sensing element 30, theoscillator 42 coupled with the sensing element 30, a divider, and aselector. The oscillator 42 oscillates at a frequency that is indicativeof the percentage of the measuring range of the sensing element 30 thatis covered by the target tube 60. Because the oscillator frequency maybe in the megahertz (MHz) region, a divider can be used to increase theperiod for easier measurement. The selector allows the selection of anappropriate number of stages of division. A control section includes themicroprocessor 46 that receives the second frequency signal 45, selectsthe division amount, receives temperature information, and sendscalibrated digital information to a digital to analog (D/A) converter. Aprogramming port connects with the microprocessor 46 for loading of itsoperating firmware, and a calibration port is also connected forcalibration in order to provide the desired scaling of the output. Anoutput section receives an analog output from the D/A converter andproduces the output voltage or current. The output section also includesprotection of the output circuit from damage due to transient voltagesor overcurrent. A digital output is also an option.

Relative to the conventional electronics module 41, the '541 patentillustrates and describes a simplified electronics module. Thesimplified electronics module is illustrated in FIG. 4 of the '541patent and described in the accompanying text of that patent. Thedisclosure of the '541 patent is incorporated into this document byreference in its entirety.

As shown in FIG. 5 of this document, a single coaxial cable 200 could belocated between, and used to interconnect, the resonant oscillator 42and the frequency meter 44. In this configuration, only the resonantoscillator 42 is attached directly, via the short wire 47, to thesensing element 30. Such a configuration allows for a partially removedelectronics module 49 comprising the frequency meter 44 and themicroprocessor 46, but devoid of the resonant oscillator 42. Theelectronics module 49 is called “partially removed” because some of theelectronic components are separated (i.e., removed) from the sensingelement 30 by a significant distance defined by the length of thecoaxial cable 200. The electronics module 49 (including all of itscomponents) is attached directly to the coaxial cable 200 and onlyindirectly to the sensing element 30 through the resonant oscillator 42.Because the resonant oscillator 42 must be in the same environment asthe sensing element 30 in the configuration of FIG. 5, however, the mainadvantages of incorporating the coaxial cable 200 into the configurationare not achieved.

Preferably, the addition of the coaxial cables 200 to the sensor 100allows all of the electronics of the sensor 100 to be located at asignificant distance from the sensing element 30. Therefore, as statedabove, the addition of the coaxial cables 200 to the sensor 100 allowsfor placement of the sensing element 30 in a harsh environment or in alocation that is difficult to reach. As shown in FIG. 6, one coaxialcable 200 is connected directly to each one of the helices 34, 36 of thesensing element 30. The coaxial cables 200 are located between, and usedto interconnect, the sensing element 30 and the resonant oscillator 42.Such a configuration allows for a remote electronics module 70comprising all of the electronic components (the resonant oscillator 42,the frequency meter 44, and the microprocessor 46) to be located asignificant distance (i.e., remote) from the sensing element 30. Thatdistance is defined by the length of the coaxial cables 200. Theelectronics module 70 (including all of its components) is attacheddirectly to the sensing element 30 through the coaxial cables 200.Because all of the electronics of the sensor 100 are remote from theenvironment of the sensing element 30 in the configuration of FIG. 6,the main advantages of incorporating the coaxial cables 200 into theconfiguration are achieved.

According to an embodiment of the present invention illustrated in FIG.7, an apparatus for the measurement of linear position has at least oneelectromagnetic sensing element 30 connected to an RF or microwaveoscillator 4 and to a measuring circuit 5. The measuring circuit 5includes at least a first converter 14 for converting the oscillatingsignal to a frequency value and then passing on this information to amicroprocessor. The microprocessor converts the data into an electricalsignal, such as a DC (direct current) voltage or a digital number, and asecond converter 15 converts the electrical signal into a measurement oflinear position.

The sensing element 30 is formed by a section of a coupled slow-wavestructure 6 formed by impedance conductors 7 and 8 (e.g., inner andouter helices 34 and 36, respectively) installed on a dielectric base 13(e.g., electrically insulated rod 32) disposed along an axis 13 a. Theslow-wave structure 6 is connected by terminals 16, 17, through thecoaxial cables 200, to the oscillator 4. The slow-wave structure 6 alsohas terminals 18 and 19, which can be connected to additional coaxialcables 200 and can be open, loaded with an impedance, shorted, orconnected to the measuring circuit 5.

Each of the impedance conductors 7, 8 are curled into a helix or spiralwith opposing directions of winding. The pitches of the impedanceconductors 7, 8 are chosen to provide the desired distribution of themagnetic field within the distance between the sensing element 30 and atarget 3. In the case of a set of coaxial helices, the spacing betweenthe helices 34, 36 and the cylindrical target 3 should be less than theaverage radius of the helices 34, 36.

The oscillator 4 excites a sine wave or complex signal in the sensingelement 30. This signal reflects from the sensing element 30 or passesthrough it, causing a voltage difference to appear across the terminals16, 17 and the terminals 18, 19, the magnitude of which depends on thedistance to the target 3. Therefore, a change in the distance leads to achange in the voltage differences, and that in turn leads to a change inat least one parameter of the sensing element 30. The parameter of thesensing element 30 that changes, and is measured, can be the impedance,resonant frequency, phase shift, or the like.

The frequency of the oscillator 4 can be constant, or it can bevariable, depending on the measured parameter and the circuit used. Witha constant frequency, the signal from the oscillator 4 can be splitbetween two paths of a bridge circuit. One path is loaded by the sensingelement 30, while the second path is loaded by a reference load. Thevoltage difference between the signals of the two paths is determinedand amplified by a differential amplifier and can be used to indicate asignal depending on the impedance of the sensing element 30.

With variable frequency operation, the sensing element 30 is connectedin the feedback circuit 27 of the oscillator 4, thereby changing itsfrequency in response to changes in the measured position (FIG. 7).Other measuring circuits can be used for converting the electromagneticparameters of the sensing element 30 into a reading of linear positionand related indications. In all versions of measurements according tothe present invention, at least one coupled slow-wave based sensingelement and at least one electrically conductive target 3 are used. Insome cases, an electrically conductive surface of an object 2 can beused as the target 3.

FIG. 8 demonstrates yet another embodiment of the present invention. Inthis embodiment, a metal tube with an internal diameter is the movabletarget 3. The sensing element 30 is connected into a feedback circuit ofa Pierce oscillator 4, the operating frequency of which is used toindicate the measured linear position, or measured distance. For adistance in the range between 0 and 100 mm, the frequency will increaseapproximately proportionally to the linear position. The sensitivity isvery high (approximately 56 kHz per millimeter).

In some applications it is desirable to deploy multiple sensors makingidentical measurements of the same characteristic (e.g., distance ormotion or level). In a dual redundant system (two sensors), when bothsensors produce the same result the data are assumed to be accurate. Ifthe two sensors disagree, however, then the data from both sensors aresuspect and must be disregarded. With a triple redundant system (threesensors), when at least two of the sensors agree, the system cancontinue operation based on the values of the two sensors in agreementand the value from the third sensor is ignored.

With conventional LVDT and magnetostrictive sensors, redundant systemscan be deployed only by installing two or three separate sensors andthen mechanically coupling each of them to the same target (e.g., amovable object). This linkage introduces errors due to differences inalignment, backlash (lost motion), and other imperfect attributes of themechanical coupling. By comparison, the sensor 100 has the ability tomake a true double or triple redundant measurement without any need forseparate linkages and only a small increase in the physical size of thesensor 100.

The sensor 100 has few components and is extremely robust and reliable.Nevertheless, in some critical applications redundancy may still bedesirable. The ability of the sensor 100 to perform a true double ortriple redundant measurement with only a slight expansion of itsphysical size is both practical and elegant. In these embodiments, thesensor 100 implements two or three position-sensing sets of helices 34,36 (each set of helices 34, 36 in the sensing element 30 forming aresonator), and one target 3, in the same physical space as a single setof helices 34, 36 and target 3.

For a double redundant system, the nonconductive shaft or electricallyinsulated rod 32 of the sensor 100 can be extended and a second coilwound on this extension, as shown in FIGS. 9 and 10. The first set ofhelices 34, 36 form a first coil 53; the second set of helices 34, 36form a second coil 57. The sensor 100 having two coils 53, 57 can besaid to have two channels of measurement and can provide dual-redundantmeasurement of the same position. FIG. 9 shows two sets of helices 34,36 forming the two coils 53, 57 aligned linearly (or, in-line) withone-another. Alternatively, the two coils 53, 57 can be alignedconcentrically, with one coil surrounding the other, and the target tube60 moving in the annular space between them, as will be shown for thethird channel of FIG. 12.

In the two-channel version of FIGS. 9 and 10, a pair of coaxial cables200 are connected to the first set of helices 34, 36 that form the firstcoil 53 within the housing 10 and a second pair of coaxial cables 200are connected to the second set of helices 34, 36 that form the secondcoil 57 within a housing 10A. The coaxial cables 200 connect on theirends opposite the first and second coils 53, 57 with one or moreelectronic modules. As shown in FIG. 10, the target tube 60 position isnear 100% coverage of the first coil 53, toward the left, and near 0%coverage of the second coil 57. If the target tube 60 is moved to theright, then the second coil 57 will continue to increase being coveredtoward 100%, as the first coil 53 will continue to be uncovered toward0%.

At first glance, the configuration of the two-channel version of thesensor 100 shown in FIGS. 9 and 10 appears to double the length of thesensor 100. But that observation is not true. The conductive target tube60 that covers the sensing element 30 must have an extra space equal tothe length of the sensing element 30 into which it is withdrawn when thecoil 53, 57 is uncovered. This configuration is shown in FIG. 11 withthe target tube 60 at full extension.

When two sensing elements 30 are located on the same electricallyinsulated rod 32, the single target tube 60 serves two functions. As thetarget tube 60 uncovers the first coil 53 it simultaneously covers thesecond coil 57. Thus, a redundant sensor 100 requires no additionalspace. More important, however, both coils 53, 57 measure the samemoving surface with no requirement for mechanical attachments.

The sensor 100 can easily be extended to a triple redundant system byadding a third coil 59. The third coil 59 is wound on a larger diameterhollow non-conductive tube 61 that fits over the moving target tube 60.The target tube 60 moves in an annular space between the first coil 53and the third coil 59. The first coil 53 is barely visible in FIG. 12,because it is located concentrically within the third coil 59. Thetarget tube 60 covers more or less of the first coil 53 and the secondcoil 57 as it is moved from left to right, as explained above for FIGS.9 and 10. The second coil 57 is positioned in-line with the first coil53. The outer third coil 59 will resonate (albeit at a differentfrequency from the first coil 53 and the second coil 57) in response tomovement of the target tube 60. This configuration is shown in FIG. 12as an exploded view showing the three coils 53, 57, 59 and the targettube 60 that forms the actual measurement point 63.

The sensor 100 illustrated in FIG. 12 can provide three, separatemeasurements based on the same moving target tube 60. All three coils53, 57, 59 can be calibrated at the same time. There is some penalty insize for having the third coil 59, in that the diameter of the totalsensor 100 has increased slightly, although the length (which is usuallythe more important parameter) remains unchanged.

EXAMPLES

The following examples are included to more clearly demonstrate theoverall nature of the invention. These examples are exemplary, notrestrictive, of the invention.

Linear measurement tests were performed using the single non-redundantsensor 100 having coaxial cables 200 as described above. The length ofthe coaxial cables was about 4.8 meters (16 feet). Specifically, SensorSerial Number 19142Re-Test, Part Number A5-8.0-M12-R4-R4-B0139, wastested. Table 1 shows the test results.

TABLE 1 Position (Inches) Output (Volts) Error % of Full Scale 0.0530.088 −0.008% 0.266 0.352 −0.034% 0.533 0.681 −0.079% 0.800 1.021−0.012% 1.067 1.353 −0.028% 1.333 1.688 −0.011% 1.600 2.020 −0.027%1.867 2.354 −0.022% 2.133 2.693 0.032% 2.400 3.023 −0.004% 2.667 3.3580.012% 2.934 3.695 0.047% 3.200 4.029 0.051% 3.467 4.366 0.084% 3.7344.698 0.071% 4.001 5.028 0.033% 4.268 5.366 0.079% 4.535 5.698 0.060%4.801 6.026 0.008% 5.068 6.356 −0.031% 5.335 6.691 −0.016% 5.602 7.023−0.032% 5.868 7.355 −0.045% 6.135 7.691 −0.022% 6.402 8.026 −0.007%6.668 8.357 −0.032% 6.935 8.690 −0.036% 7.202 9.023 −0.041% 7.469 9.360−0.007% 7.735 9.694 0.001% 8.002 10.029 0.015%

FIG. 13 graphically reflects the test results as a plot of output of thesensor 100 in volts against the distance measured in inches. Theabscissa and the ordinate are the horizontal and vertical axes,respectively, typically the x-axis and y-axis of a two-dimensionalgraph. FIG. 13 shows the output of the sensor 100 on the ordinate andthe distance on the abscissa. A best fit line can be calculated for thedata according to the equation for a straight line, Y=M(X)+B, where M isthe slope of the line and B is the intercept of the line with theordinate. For the data provided above, the slope, M, is 0.800 and theintercept, B, is −0.018 so that the best fit line has the equationY=0.800(X)−0.018.

FIG. 14 graphically reflects the non-linearity of the test results as aplot of full scale error percent against the distance measured ininches. The non-linearity fluctuates within a band of ±0.082%. Thelinear error is a measure of the accuracy of the sensor 100.

The graph of FIG. 15 shows a series of three tests run from −70 to 180°C. for the sensor 100. The sensing element 30 used in this exampleincludes magnet wire wound on a ceramic rod held together with Kapton®tape. (Kapton® tape is made from Kapton® polyimide film with siliconeadhesive. Such tape is compatible with a wide temperature range as lowas −269° C. and as high as 260° C. Kapton is a registered trademark ofE.I. du Pont de Nemours and Company.) As can be seen in the graph ofFIG. 15, reproducibility of the sensor 100 is excellent and the totalerror in this temperature range is less than ±1% of full scale. The testdata can be fitted with a polynomial equation, specifically, in thisexample, the equation y=−1E−10x⁴ +3E−08x³+2E−07x²−0.0002x+0.0011. Thedotted curve for the polynomial equation is included on the graph ofFIG. 15. R² for this fit is 0.9887. (R² is a statistical measure of howclose the data are to the fitted regression curve. It is known as thecoefficient of determination, or the coefficient of multipledetermination for multiple regression.) The fitting allows correction ofeven this small error down to virtually zero by monitoring thetemperature and correcting the output. The errors are not linear becausethe expansion and contraction of the materials involved include volume,area, and linear effects but are reproducible.

The graph of FIG. 16 reflects data for high-temperature operation of thesensor 100 up to 580° C. The magnet wire is replaced in this examplewith bare copper wire and ceramic tape as insulation between the coilsof the sensing element 30. As can be seen, the maximum error stillremains within ±1% error of full scale and can also be fitted with apolynomial equation. Specifically, in this example, the equation isy=−2E−11x³+4E−08x²−4E−05x+0.0087. The dotted curve for the polynomialequation is included on the graph of FIG. 16. R² for this fit is 0.9997.

The graph of FIG. 17 combines the data of the first two graphs of FIGS.15 and 16, respectively. FIG. 17 shows that the minimal errors arecontinuous and reproducible from −70° up to 580° C. A temperature of580° C. is the maximum for copper wire because copper rapidly oxidizesat higher temperatures. Higher and lower temperatures can be achieved bysubstituting different metals for the coils of the sensing element 30such as tungsten, molybdenum, and platinum which would extend thetemperature range from near absolute zero to over 3,000° C. Althoughsensors have not been tested in these extreme temperature ranges,technology for making such coils is readily available and in common use.The resistance of most metals changes with temperature but this changedoes not affect the output of the sensor 100. (This temperaturevariation could be used as a crude temperature sensor but is notreliable.) This is due to the fact that in a resonant circuit the changein resistance only affects the amplitude of the signal and not thefrequency. The sensor 100 uses the frequency for measurement and theamplitude is not a factor so long as the amplitude stays above a certainthreshold.

The sensor 100 is suitable for a wide variety of applications. Among themore important applications are high-temperature operations such aspower plants, jet engines, ovens, and combustion engines.Low-temperature operations (less than about −60° C.) are also important,especially in aviation. The various moving devices on airplanes areconsistently exposed to low temperature when cruising at high altitude.In robotics, the ability to place many very small sensors 100 throughoutthe robot and place the control circuits in a central location isbeginning to mimic the human body. Exploring for oil and gas requiresmaking deep measurements in the earth, where temperatures can exceed225° C. In essence, the sensor 100 can replace LVDT sensors in allapplications currently monitored using LVDT sensors—especially in thoseapplications above the absolute temperature limit for LVDT sensors ofabout 500° C.

Focusing on jet engine applications as an example, the sensor 100 isespecially well-suited to make proximity measurements. A flat coil whenexcited will change its resonance frequency as it approaches any metalconductive surface. This measurement is very important in measuring thegap between turbine blades tips and the outer housing of jet engines andcan only be performed now at room temperature. In an operating enginethe temperatures reach thousands of degrees and few, if any, electroniccomponents can survive such temperatures. By using the sensor 100, witha sensing element 30 made with platinum helices 34, 36 (tungsten helices34, 36 are used for less harsh environments) and connecting coaxialcable 200, an accurate gap measurement can be obtained despite suchtemperatures. It is then possible to make this measurement in anoperating engine.

For example, because the bandwidth of the sensor 100 is several Mhz, thesensor 100 can be used to measure the gap of each turbine blade tip asit passes the sensor 100 during operation of the jet engine. A modifiedsensor 300 useful to measure that gap is illustrated in FIGS. 18A, 18B,and 18C. FIG. 18A shows the helices 34, 36 as wound into coils. Thecoils are then folded over each other and imbedded into a ceramic body302 as shown in FIG. 18B. FIG. 18C is a side view of the modified sensor300 as located in the cowling wall with the turbine blade 304 above thesensor 300 and two coaxial cables 200 leading from the coils andextending out from the ceramic body 302. The ceramic body 302 functionsas an insulator.

The top of the modified sensor 300 must be flush with the interiorsurface of the engine cowling which means that a recess would have to beformed in the outer wall and holes (for the feedthrough) drilled throughto the outside. The ability to produce precision ceramics for theceramic body 302 means that the sensor 300 can conform to the curvatureof the cowling without additional grinding steps. With ceramic being theonly exposed surface to the engine environment, and with the electronicslocated remotely, the modified sensor 300 can withstand the extremechemical, temperature, vibration, and pressure conditions of anoperating jet engine and should easily meet a 10,000 operating hoursrequirement. The weight of the modified sensor 300 is a few ounces.

Because the frequencies and bandwidth being measured are in the MHzregion, the modified sensor 300 can detect a single blade 304 passingthe modified sensor 300. This will produce a gap measurement for eachblade 304 although it may require averaging over many cycles. Becauseevery blade 304 will have a slightly different geometry, it may bepossible to develop a “map” of all the blades 304 to identify theabsolute blade number. This could be very valuable for monitoringchanges in blade lengths while the engine is in operation.

Another application for the sensor is firing ceramic components in akiln. Ceramic parts are made by molding ceramic particles with a binder(usually a wax or oil) into a fixed shape and then firing the moldedpart at a high temperature. During the firing process, the part shrinksas the binder boils off and the particles bind together eliminating thespaces between them. This shrinkage is critical to producing good partsand parts that are not completely fired will be defective. Whendeveloping a firing pattern for a certain ceramic part, experiments aredone to see how much time and temperature are needed to produce thefully fused part. Because temperature cycle times for firing range fromseveral hours to several days, these are long and laborious experiments.Even when completed, firing schedules are usually extended to ensurethat full shrinkage has been obtained.

The sensor 100 can measure the shrinkage when the sensor 100 isinstalled in the kiln at the elevated temperatures. A simplespring-loaded plunger attached to the target and pushing against thepart to be fired can measure the shrinkage with good accuracy. Themechanical parts of the sensor 100 (spring, target tube, etc.) wouldhave to be made, of course, out of a refractory metal such as tungsten,platinum, or molybdenum but such machine parts are routinely used inhigh-temperature ovens. The ability to determine that a part has fullyshrunk would offer an enormous economic saving for ceramicmanufacturers. Use of the sensor 100 would allow manufacturers toshorten or even eliminate much of the experimentation and can reducecycling times. There is nothing available that can make this kind ofmeasurement at these elevated temperatures.

Superconductivity is a phenomenon that occurs at very low temperatures.At such temperatures, certain metals lose all their resistance tocurrent flow. In addition to having this unique characteristic, anotherfeature of superconductivity is that the magnetic field generated byflowing current is excluded from the body of the metal. This feature isuseful in making superconducting electromagnets which are currently usedin almost all MM scanners. The sensor 100 does not generate an externalmagnetic field, however, given the opposite windings of the coils of thesensing element 30. Each winding produces a magnetic field but inopposite directions, so they cancel each other out. The lack of anymagnetic field outside a superconducting sensor could be advantageous tocertain measurements made at these extremely low temperatures. Magneticfields are difficult to shield and can cause interference in low levelmeasurements. There is even the possibility that using the sensor 100 ona small scale could be beneficial in quantum computers where thepresence of a strong magnetic field would be detrimental. The ability tohave a sensor 100 that can measure position while producing no externalmagnetic field could be advantageous in many applications.

In summary, the sensor 100 is a relatively simple, economical, andcompact device that can make accurate and reliable linear measurementsin almost any environment using coaxial cables 200. Although thedisclosure focuses on linear sensors, the concepts disclosed above alsoapply to other configurations of the sensor 100, including radial orrotary motion, proximity, and liquid levels. The coil configuration isdifferent for each measurement but the electronics are the same.

Obtaining an output proportional to a frequency rather than an analogvoltage or current has many advantages. The first advantage is thatfrequency is a digital signal so the amplitude of the signal is notimportant as long as it is above the threshold of detection. Allconventional sensors produce an analog signal which makes themsusceptible to noise, attenuations, and other distortions. Many othersensors require that the analysis electronics be close to the sensor orif the electronics can be located remotely, they have extensivecorrection codes to compensate for these analog errors.

The sensor 100 can be used to measure, among other parameters, linear orrotary position, gaps, and liquid levels. An important function of thesensor 100 is its use as an absolute linear encoder which can measurelengths from a few millimeters up to one meter while maintainingaccuracies in the micron region. The output of the sensor 100 isfrequency, a digital signal, and is not sensitive to noise andattenuations in the signal path. The sensor 100 requires only a standardfrequency meter to interpret the position of an object 2. The sensor 100requires no magnets or magnetic material and is insensitive to externalmagnetic fields. The sensor 100 can be manufactured with redundancy inthe same physical package with only a small increase in diameter (butnot length) and can be made triple redundant allowing majority logic.The same, unchanged length can be achieved for both double and tripleredundancy embodiments of the sensor 100.

The sensor 100 described above offers several advantages when comparedto the existing competing technology of the LVDT. The sensor 100 farexceeds the performance of an LVDT as it is smaller, more accurate, andcheaper to manufacture. The LVDT can be viewed simply as a transformerwith a moveable core. As the core moves in and out (which is the motionbeing measured), the transformer output voltage between the primary andsecondary coils changes. If the coils are properly wound, the change inoutput voltage is linear (or almost linear) with the motion. Electronicsare required to control the LVDT and must compensate for the effects ofthe wiring between sensor and electronics. The LVDT upper temperaturelimit is nominally 200° C. but that can be raised at great cost andeffort to about 500° C. The sensor 100 has been operated to 580° C. andexpectations are that the temperature limit for the sensor 100 isconsiderably higher (i.e., about 1,000° C. or more).

The electronics that produce an input voltage to the primary coil andmeasure the output of the secondary coil for the AC version of the LVDTcan be located remotely. Although the electronics can be locatedremotely, they are very complicated and have to compensate for thelength and types of connecting cables. This is due to the fact that theprocess measures the amplitude of the signal which is attenuated bydistance. The sensor 100 measures frequency, which is not affected bydistance, so no compensation is required.

Another disadvantage of the LVDT is what is referred to as “thestroke-to-length ratio.” In order to measure a linear distance of Xinches, the sensor must be 3X inches long which defines astroke-to-length ratio of 3:1. The sensor 100 offers a more efficient2:1 ratio. In addition, the sensor 100 can be made redundant in asimpler and more reliable manner than the LVDT. A third disadvantage ofthe LVDT is the large number of windings in a coil and small diameter(fragile) of the wire used. Certain embodiments of the sensor 100 userugged large diameter wire, and much fewer turns in the coil.

Finally, the linear error (accuracy) of an LVDT rarely is less than0.25% of full scale. The sensor 100 typically reduces the error to0.05%. In theory, the sensor 100 can reach accuracies of parts permillion but this is rarely required.

In addition to being able to operate in environments of high and lowtemperatures (less than about −60° C. to more than about 1,000° C.), athigh radiation levels (mega rads), under high pressure, despitevibration or shock, and in the presence of caustic chemicals or steam,the sensor 100 is inherently “intrinsically safe.” Intrinsically safemeans that, in an explosive atmosphere, the current and voltage flowingin the sensor 100 lack sufficient energy to produce a spark under anyconditions of operation. The current and voltage in the sensing coilsare an order of magnitude below the threshold of producing a spark andcould be reduced even further if necessary. The sensing coils can alsobe made smaller, allowing the sensor 100 to be used in very tightapplications (i.e., applications having significant geometricallimitations).

Although illustrated and described above with reference to certainspecific embodiments and examples, the present invention is neverthelessnot intended to be limited to the details shown. Rather, variousmodifications may be made in the details within the scope and range ofequivalents of the claims and without departing from the spirit of theinvention. It is expressly intended, for example, that all rangesbroadly recited in this document include within their scope all narrowerranges which fall within the broader ranges.

What is claimed:
 1. A sensing apparatus for use in harsh environments tomeasure a characteristic of an electrically conductive target, theapparatus comprising: a sensing element formed as a section of a coupledslow-wave structure including at least two impedance conductors eachcurled into a helix or spiral with opposing directions of winding arounda dielectric base to form a resonator, the sensing element providing asan output signal a digital frequency that depends on the value of themeasured characteristic of the target; a target tube formed of anelectrically conductive material and configured to move over the sensingelement, covering and uncovering portions of the sensing element; anelectronics module including electronic components configured to receivethe output signal from the sensing element and display thecharacteristic of the target; and at least two coaxial cables eachhaving a length and being connected to one of the at least two impedanceconductors on one end and to the electronics module on the other end,with the length of the coaxial cables separating the electronics modulefrom the sensing element by a distance sufficient to avoid exposing theelectronics module to the harsh environments.
 2. The sensing apparatusaccording to claim 1 wherein the at least two impedance conductors areeach constructed of high-temperature, flat, insulated wire.
 3. Thesensing apparatus according to claim 1 wherein the at least twoimpedance conductors are each constructed of conductive wire and thesensing element further includes ceramic tape as insulation between theat least two impedance conductors.
 4. The sensing apparatus according toclaim 1 wherein the at least two impedance conductors are eachconstructed of a metal selected from tungsten, molybdenum, or platinum.5. The sensing apparatus according to claim 1 wherein the length of theat least two coaxial cables is at least about three meters.
 6. Thesensing apparatus according to claim 1 wherein the length of the atleast two coaxial cables is at least about six meters.
 7. The sensingapparatus according to claim 1 wherein the electronics module isattached directly to the sensing element through the at least twocoaxial cables.
 8. The sensing apparatus according to claim 1 whereinthe electronic components include a resonant oscillator, a frequencymeter, and a microprocessor.
 9. The sensing apparatus according to claim1 wherein the apparatus is configured to make double or triple redundantmeasurements.
 10. The sensing apparatus according to claim 9 wherein theapparatus is configured to make double redundant measurements, thedielectric base has an extension, and two additional impedanceconductors are wound on the extension.
 11. The sensing apparatusaccording to claim 10 wherein the at least two impedance conductors andthe two additional impedance conductors are aligned linearly.
 12. Thesensing apparatus according to claim 10 wherein the at least twoimpedance conductors and the two additional impedance conductors arealigned concentrically.
 13. The sensing apparatus according to claim 9wherein the apparatus is configured to make triple redundantmeasurements, the dielectric base has an extension, a first set of twoadditional impedance conductors are wound on the extension. a largerdiameter hollow non-conductive tube fits over the moving target tube, asecond set of two additional impedance conductors are wound on thelarger diameter hollow non-conductive tube, the at least two impedanceconductors and the first set of two additional impedance conductors arealigned linearly, and the at least two impedance conductors and thesecond set of two additional impedance conductors are alignedconcentrically.
 14. The sensing apparatus according to claim 1 whereinthe apparatus is configured to operate in a harsh environment having atemperature of at least 225° C.
 15. The sensing apparatus according toclaim 1 wherein the apparatus measures linear or rotary position. 16.The sensing apparatus according to claim 15 wherein the apparatusmeasures lengths from one millimeter up to one meter while maintainingaccuracies in the micron region.
 17. The sensing apparatus according toclaim 15 wherein the apparatus has a stroke-to-length ratio of 2:1. 18.The sensing apparatus according to claim 15 wherein the apparatus has alinear error of less than 0.05% of full scale.
 19. The sensing apparatusaccording to claim 1 wherein the apparatus is intrinsically safe.
 20. Asensing apparatus for use in harsh environments to measure acharacteristic of an electrically conductive target, the apparatuscomprising: a sensing element formed as a section of a coupled slow-wavestructure including at least two impedance conductors each curled into ahelix or spiral with opposing directions of winding around a dielectricbase to form a resonator, the sensing element providing as an outputsignal a digital frequency that depends on the value of the measuredcharacteristic of the target; a target tube formed of an electricallyconductive material and configured to move over the sensing element,covering and uncovering portions of the sensing element; an electronicsmodule including a resonant oscillator, a frequency meter, and amicroprocessor configured to receive the output signal from the sensingelement and display the characteristic of the target; and at least twocoaxial cables each having a length and being connected to one of the atleast two impedance conductors on one end and to the electronics moduleon the other end, so that the electronics module is attached directly tothe sensing element through the at least two coaxial cables, with thelength of the coaxial cables separating the electronics module from thesensing element by a distance sufficient to avoid exposing theelectronics module to the harsh environments, wherein the apparatus isintrinsically safe and is configured to operate in a harsh environmenthaving a temperature of at least 225° C.